I've been looking at Zookeeper recently and wondered whether anybody was using it currently and what they were specifically using it for storing.
The most common use case is for configuration information, but what kind of data and how much data are you storing?
HBase uses Zookeeper for coordinating activities its "head node" was responsible for prior to the current version. The move to using Zookeeper means the central control is no longer a single point of failure.
Zookeeper is very versatile; here is an example of using it to build a distributed concurrent queue:
You can of course also use it to create resource locks, etc, in a distributed system.
The Apache CXF implementation of DOSGi uses zookeeper for its service registration repository. Individual containers have a distributed software (dsw) bundle that listens for all service events and when a service status changes that has a property indicating distribution. The dsw talks to the discovery bundle which, in the reference implementation case, uses zookeeper to store service as ephemeral nodes. Other instances will look for changes to the node structure and register proxies on their local systems. The end result is you can code to plain OSGi and end up with transparent distribution.
Here's some detail on how HBase uses ZooKeeper, including information on how they intend to use it in future: http://bit.ly/cWGUSa Generally they use it for eliminating SPOF on the region servers via Leader election implemented using ZooKeeper.
Solr is also working to integrate ZooKeeper, here's how they are using it: http://bit.ly/95rhUJ Here you can see they are using for dynamic config, sharding, SPOF elimination (master/slave election), rebalancing, etc...